When you replace the lower anti-collision light assy (same assy as on top of fuselage) on a NG, at the end you have to remove the plug or screw from the middle, to prevent water collecting inside the light assy. Often this plug is forgotten to take out, and the glass will collect water until the light stops working on a certain moment. Once we had a completely full light assy, so you could not see the water level, and of course the light stopped working. So remember to take out the plug or screw as showed in the example photo’s. See amm 33-44-02-960-802 for full instructions.